See slope: https://en.wikipedia.org/wiki/Slope

It isn't customarily used for angles (those get Greek letters).

The m stands for mystery.

Edit: ah, but I see that this prefixed m for "measure" is also used sometimes. It appears at https://en.wikipedia.org/wiki/Angle#Combining_angle_pairs though I'm not sure why it's necessary. Maybe because you want to clarify absolute magnitude is meant, to avoid adding negative values.